The present invention relates generally to methods and systems for obtaining ultrasound images, and more particularly, to such methods and systems that provide real-time ultrasound images having clinical quality.
An ultrasound system can typically include a transducer array, a signal processing unit and a display. The transducer elements generate ultrasonic waves, transmit the waves into a region to be imaged, and receive returning echoes, generated in response to the transmitted waves, by one or more scatterers in the region. The signal processing unit utilizes the echoes to construct an image of the scatterers, which can then be presented to a viewer on the display.
In traditional ultrasound systems, narrow beams are employed for image acquisition. In many such systems, the transducer elements transmit identically-shaped pulse signals which are delayed relative to each other to ensure that the pulses arrive at a desired focal point at the same time, thus forming a beam in a particular direction. During the receiving step, the echoes generated in response to the pulses are similarly delayed so that at any particular time, the echo signals sent by the transducers to the processing unit correspond to signals generated at the same point along the beam. The image values corresponding to scatterers located along the beam direction are set to the sum of the intensities of the respective echo signals. This procedure is often referred to as xe2x80x9cdelay-and-sumxe2x80x9d, or xe2x80x9cbeamformingxe2x80x9d. An image of a selected region is constructed by repeating this process along a number of transmitted beam directions. The system component, typically hardware, that performs delaying and adding of the echo signals to isolate the scatter properties in a particular location is called a xe2x80x9cbeamformerxe2x80x9d.
In most traditional ultrasound systems, the transducer elements are arranged along a single straight or curved line, which confines the transmitted waves to an imaging plane. A resulting image corresponds to a cross-section of an imaged object along the imaging plane. More recently, matrix (2-dimensional) transducer arrays have been introduced that allow full volumetric imaging. Alternatively, a linear array can be moved/rocked to transmit pulses in all directions in a given volume.
The data collection time in the systems described above is proportional to the number of beams required to generate the image. The number of beams required to generate a volumetric image is equal to the square of the number of beams required to form a planar image of the same resolution. For example, to extend a two-dimensional 64-beam image into three dimensions (3D) while maintaining the same resolution, 64xc3x9764=4,096 beams are needed. Similarly, extending a 128-beam image into 3D requires 128xc3x97128=16,384 beams. Hence, a transition from planar to volumetric imaging can result in approximately two orders of magnitude increase in the amount of data and the acquisition time. Since the time of each transmit-receive iteration (i.e., transmitting a single beam and receiving the echoes from the scatterers in the selected region) is determined by the speed of sound in the region to be imaged (e.g., tissue), the number of beams that the system can transmit and receive in any given time is inherently limited (approximately 5000 per second). At real-time frame rates (e.g., 30 frames per second), this corresponds to approximately 150 beams per image, which is insufficient for volumetric imaging.
Thus, there is a need for improved ultrasound imaging methods and associated systems. There is also a need for such ultrasound imaging methods and systems that allow efficiently generating ultrasound images in real-time.
The invention provides a method of generating an ultrasound image of a plurality of scatterers disposed in a target region by constructing response functions for each of a plurality of transducers for a given ultrasound interrogation pattern and a given distribution of scattering media. The interrogation pattern can be selected to include a set of unfocused ultrasound waves generated by one or more of the transducers. The phrase xe2x80x9cunfocused ultrasound wavexe2x80x9d, as used herein, refers to one or more ultrasound waves that have not been designed, for example, by selection of their relative phases, to substantially interfere constructively in a selected region. The interrogation pattern is transmitted into the target region, and the transducers are utilized to detect echoes generated by scatterers in the target region in response to the interrogation pattern.
An image of the scatterers is then globally constructed based on comparison of the detected echoes and echoes predicted by the response functions. The term xe2x80x9cglobally constructing an imagexe2x80x9d, as used herein, refers to computing the ultrasound image by mathematically processing echoes received from any part of an entire portion of the target region that is illuminated by the unfocused transmitted ultrasound waves, including any interferences among these echoes, without the need for beamforming. Hence, the method of the invention generates an ultrasound image of a selected target region without utilizing beamforming either in transmission of ultrasound waves into a target region or in detection and processing of echoes generated by scatterers in that region in response to the transmitted waves.
In a related aspect, an echo signal fn(t) detected by the n-th transducer of a plurality of transducers is defined in accord with the relation:
fn(t)=∫xcexdFn(t, v)dv
wherein xcexd represents a selected region to be imaged, v represents a particular location in the selected region xcexd, and Fn(t, v) represents a function predicting echo signal that is reflected by scatter at point v and detected by the n-th transducer.
In many embodiments of the invention, a linear model is utilized for predicting echoes detected by each transducer. For example, an echo signal fn(t) detected by n-th transducer can be defined in accord with the relation:
fn(t)=∫xcexdBn(t, v)s(v)dv
wherein s(v) represents a scattering parameter of a scatterer positioned at point v in the selected region xcexd, Bn(t, v) represents a linear response function associated with the n-th transducer element corresponding to a point v in the selected region xcexd.
In a related aspect, the echoes detected by the transducers are discretized. This discretization process can be accomplished uniformly, for example, by sampling and digitizing each echo signal at uniform temporal intervals. Alternatively, the echo signals can be discretized non-uniformly, for example, by sampling and digitizing each echo signal at temporal intervals having different durations. For example, an echo signal associated with the n-th transducer fn(t) can be discretized into a plurality of echo signals fn(k), each of which is defined in accord with the relation:
fn(k)=∫xcexdBn(k, v)s(v)dv
wherein k is an index representing a discrete echo sample, ranging from 1 to K, and B(k, v) is the response function associated with the n-th transducer discretized using the same time intervals as the detected echo signal.
In some embodiments, the target region can be represented as a plurality of discrete portions. The discrete portions can have the same or variable sizes. Further, the discrete portions can be distributed through the target region in a uniform or non-uniform manner. In such a case, an echo fn(t) associated with the n-th transducer can be defined in accord with the relation:
            f      n        ⁡          (      t      )        =            ∑              v        =        1            V        ⁢          xe2x80x83        ⁢                            B          n                ⁡                  (                      t            ,            v                    )                    ⁢              xe2x80x83            ⁢      s      ⁢              xe2x80x83            ⁢              (        v        )            
where "ugr" enumerates the discrete portions ranging from 1 to V.
In a related aspect, in a method of generating an ultrasound image as described above, the model response functions are derived based on any of computational modeling, measurements using a calibration phantom, or a combination thereof. For example, the step of deriving model response functions for the transducers can include detecting, with each transducer, an echo signal from a calibration phantom in response to pre-defined excitation signals transmitted into the calibration phantom by one or more transducers. The unfocused transmitted waves are then selected to include the pre-defined signals.
In some preferred embodiments, simplification of the functional form of a response function associated with n-th receiving transducer can be achieved by modeling an echo received by this transducer in response to an interrogation pattern generated by a plurality of transmitting transducers as a sum of echo waveforms that the n-th transducer would have received if the transmitting transducers transmitted their respective waveforms one at a time. In particular, the response function Bn(t, v) can be defined in accord with the following relation:             B      n        ⁡          (              t        ,        v            )        =            ∑              m        =        1            M        ⁢          xe2x80x83        ⁢                  B        nm            ⁡              (                  t          ,          v                )            
wherein Bnm(t,v) is a pairwise response function representing a contribution of the m-th transmitting transducer to the echo signal detected by the n-th receiving transducer. This assumption can advantageously reduce the number of calibration steps and/or simplify analytical modeling of the response function.
In a related aspect, the invention derives a model for a response function of a transducer element analytically based on the physical properties of ultrasound propagation and reflection in the target medium. According to this model, an excitation signal Em(t) applied to the m-th transmitting transducer can result in generation of an ultrasound waveform by that transducer whose amplitude can be represented by a convolution of the excitation signal Em(t) and an impulse response function hm(t) of the transducer. The amplitude of the generated waveform can be modeled as decaying linearly with the traveled distance before it is reflected by a scatterer at location v. The reflected wave can travel back to the n-th receiving transducer and impinge upon the transducer as an incoming ultrasound wave that is detected as an echo waveform. This echo waveform can be represented as a convolution of the amplitude of the incoming ultrasound wave and an impulse response function hn(t) of that transducer. Hence, the contribution of the m-th transmitting transducer to the echo detected by the n-th receiving transducer can be determined by the following relation:
            B      nm        ⁡          (              t        ,        v            )        =                    C        nm            ⁡              (                  t          -                      τ            ⁡                          (                              m                ,                v                            )                                -                      τ            ⁡                          (                              n                ,                v                            )                                      )                            τ        ⁡                  (                      m            ,            v                    )                    ⁢              τ        ⁡                  (                      n            ,            v                    )                    
wherein Cnm(t) represents a pre-defined ultrasound signal transmitted by an m-th transducer element and received by the n-th receiving transducer element (i.e., Cnm=Em*hm*hn, where * denotes convolution), xcfx84(m, v) represents a transit time of an ultrasound signal transmitted by the m-th transducer element to a point v in the target region, and xcfx84(n, v) represents a transit time of an ultrasound signal from point v to the n-th transducer element.
In a related aspect, an analytical model can be refined by employing calibration measurements. For example, the exact nature of the signal decay with traveled distance can be established using a phantom. This hybrid approach allows refining the analytical model and reducing the number of calibration measurements required to construct the response functions for all transducers.
In other aspects, in a method of ultrasound imaging according to the teachings of the invention as described above, a matrix equation is defined to relate the discrete echo signals detected by the transducers to the scattering parameters of one or more scatterers located in the discretized portions of the target region in accord with the relation:
f=Bs
wherein f is a column vector formed by concatenation of the discrete echo signals fn(k) associated with the transducers, s is a column vector that is composed of scattering parameters corresponding to one or more scatterers located in the discrete portions of the target region, and B is a matrix that is formed by concatenating the discretized response functions Bn(k, v) associated with the transducers.
In another aspect, in a method of the invention for generating an ultrasound image, subsequent to transmission of the unfocused ultrasound waves and acquisition of the echoes, the ultrasound image is globally constructed by solving a system of linear equations, such as that defined above. A solution can be obtained by utilizing a variety of techniques. For example, in some preferred embodiments, the scatter parameter vector ŝ can be obtained by linearly combining a pre-computed matrix of reconstruction coefficients (A)with the echo signals f in accord with the following relation:
ŝ=Af,
An optimal reconstruction coefficients matrix A can be defined as follows
A=(BTB)xe2x88x921BT,
wherein BT represents transpose of the B matrix, and (BTB)xe2x88x921 represents an inverse of the BTB matrix. This corresponds to minimizing the differences between the detected echo signals and the ones predicted by the model.
The computations associated with reconstructing the image can be rendered more efficient by application of a transformation to the echo signals and a compensating transformation to the matrix A to define the following relation for obtaining the scattering parameter vector s:
ŝ=(AFxe2x88x921)(Ff)
wherein F denotes a selected transformation, and Fxe2x88x921 denotes the inverse of this transformation. The transformation F is preferably selected to decrease the number of computations needed for obtaining the ultrasound image. The (Fast) Fourier transform is one example of a suitable transformation that can be utilized in the practice of the invention.
In another aspect, the invention provides an ultrasound imaging system that includes a plurality of transducers for generating and transmitting a plurality of unfocused ultrasound excitation signals corresponding to a selected interrogation pattern into a target region in which a plurality of scatterers are disposed. The transducers can also detect echoes generated by the scatterers in response to the excitation signals. The system can further include a plurality of analog-to-digital converters associated with the transducers to sample and digitize echoes detected by the transducers to generate a plurality of discrete echo signals. It should be understood that the transmitting elements can be the same or different than the receiving elements. Further, each analog-to-digital converter is not necessarily associated with only a single receiving element. That is, each analog-to-digital converter can process echoes from several transducers and vice versa. A computational module operates on the echo signals to globally construct an image of the scatterers based on comparison of the detected echoes with echoes predicted based on model response functions for each of the transducers corresponding to the interrogation pattern. For example, the computational module can construct the ultrasound image to minimize the differences between the detected and the predicted echoes.
In another aspect, an ultrasound imaging system of the invention as described above can include a memory module for storing the echo signals and/or the pre-computed reconstruction coefficients.
In a related aspect, the computational module can mathematically process the echo signals associated with the transducers in parallel to generate a plurality of intermediate output signals that can be summed to generate the ultrasound image. Alternatively, the computational module can construct different portions of the image in parallel and combine the constructed image portions to generate the entire image.
The system can also include a transformation module that receives the echo signals and applies a selected transformation, e.g., Fourier Transform, to the echo signals prior to processing of the echo signals for image construction.
In further aspects, an ultra-sound imaging system of the invention can include a user-interface having a display for presenting the constructed ultrasound image to a viewer. The system can further include a graphical object, e.g., a window, associated with the display image for selecting a portion thereof. Upon selection of an image portion, the computational module can recompute the selected portion at a resolution that is different, e.g., higher, than that of the displayed image. The re-computed image can be presented in the portion of the display associated with the graphical object, e.g., within the window utilized to select that portion. It should be understood that more than one window can be provided in the display for selecting different portions of the image. Further, a window utilized for selecting a portion of the image can have a variety of shapes, e.g., circular, rectangular, and/or a variety of sizes. Further, a graphical object utilized for selecting a portion of the image is not limited to a window. For example, in some embodiments a cursor can be provided for selecting a pre-defined area surrounding a point to which the cursor is directed.